Potential transformer fuse



Sept. 26, 1933. J. SLEPIAN El AL I POTENTIAL TRANSFORMER FUSE Filed Sept. 27, 1928 3 Sheets-Sheet 2 m I I ,4 I l I I I I I v I I I I/I I I I I N 2 I I I I I I I I I XXI /I II I II I I I I I r/ I I I I I I I 2 I P 2 Z IIII a I I I I I I I I I I I I I I I I I I I I I I I I I I II H I I I I I I I I INVENTORS Joseph Slepian and Rurl'c C.Mason BY 'ATTORNEY Sept. 26, 1933. J. SLEPIAN ET AL POTENTIAL TRANSFORMER FUSE Filed Sept. 27, 1928 3 Sheets-Sheet 5 /99.

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Amperes mym Ma/fayefibces; 01am fer Current /000 Imps. Vo/fage vs. (an-en! [afar-1' 2%; of Ho/e Diamefe) Lo /wr'l'f'fim of Carrenf INVENTORS Joseph Slepian and Ruric C-Mason BY ATTORNEY Patented Sept. 26, 1933 UNITED STATES PATENT OFFICE POTENTIAL TRANSFORMER FUSE Joseph Slepian,

Pittsburgh, Pa.,

and Ruric Application September 27, 1928 Serial No. 308,713

3 Claims.

Our invention relates to electrical circuit interrupters and more particularly to high voltage fuses for alternating-current circuits.

One object of our invention is to provide a fuse that shall be capable of interrupting high voltage alternating-current circuits.

Another object of our invention is to provide a fuse for high-voltage circuits that shall limit the rise of current when the fuse is ruptured in order to eliminate the destructive effect thereof.

It is one of the principal features of our invention that the fusible conductor is located in a narrow groove formed in a body of refractory insulating material and that the length and crosssection of this groove are correlated to the voltage of the circuit and the rated current of the fuse in a particular way which insures the effective interruption of the circuit, and which is described in detail below. The above-mentioned relationship requires in many cases that the groove shall have a considerable active length and, consequently, it is usually economical of space to give it a helical or other manifold form.

In the course of extensive experiments on the characteristics of electric arcs, it has been found that when an arc is completely confined between insulating walls, there is a critical value of current at which the arc voltage is a minimum. At all greater currents, the voltage rises with increase of current, the particular value of this critical current being dependent on the diameter of the confining recess. In the invention herein disclosed, we apply this principle to the arcs which ensue upon the melting of fuses, by so proportioning the length and cross-section of a groove in which the arc is confined that, as the current increases beyond ,the critical value aforesaid, the arc voltage comes to equal the line voltage of the circuit at a relatively moderate value. of current. Further rise of either are voltage or current is then impossible, and the rate of energy dissipation in the form of heat within the fuse casing is so moderate as to produce no destructive effects within the period of one-half cycle of the line voltage. When the arc current falls to zero in the course of the alternating cycle, the proximity of the walls of the confining recess results in such a rapid disappearance of gaseous ions capable of maintaining current flow that the arc remains extinct.

With the foregoing principles in mind, our invention will best be understood by referring to the drawings, wherein;

Figure 1 is a view, in sectional elevation, of a fuse embodying our invention,

Fig. 2 is a cross-sectional view of Fig. 1, taken on the section line II---II,

Fig. 3 is a view in sectional elevation of a fuse embodying our invention, showing a different construction from the embodiment of Fig. 1,

Fig. 4 is a cross-sectional view of Fig. 3 taken on the section line IVIV,

Fig. 5 is a view in sectional elevation of a fuse embodying our invention, showing still another construction of our invention,

Fig. 6 is a view in sectional elevation of the structure shown in Fig. 5, taken 90 therefrom,

Fig. 7 is a cross-sectional view of Fig. 6, taken on the section line V1I"VII,

Fig. 8 is a volt-ampere curve representing the characteristics of steady operation of an unconfined arc in air,

Fig. 9 is also a volt-ampere curve showing the relation of the arc voltage to the current during steady operation when the are completely fills the recess, and

Fig. 10 is an ampere or hole diameter versus arc voltage per inch curve showing a method of selecting the desired characteristics for constructing our fuse.

Our invention comprises, in general, a fuse 1 having an outercasing 2 of insulation, and end caps 3 and 4 connected by a fuse element 5. Blades 6 and 7 projecting from the end caps 3 and 4 may be employed to connect the fuse in circuit with suitable supporting clips, or alternately, blades 6 and 7 may be omitted and the caps 3 and 4 may themselves engage suitable clip terminals in a manner well known in the art.

Referring to Fig. 1, a refractory insulating core 8, which may be of soapstone, for example, is provided with a helical groove 11 in which a fuse wire 5 is wound. The core 8 is preferably closely surrounded by an insulating member 2 which may also be of soapstone. Conducting plates 12 and 95 13 close each end of the insulating member 2 and form a surface against which the fuse wire is clamped when assembling the end caps 3 and 4. It will be seen to be a result of this structure that when an overload occurs in the circuit being protected, the fuse member 5 is ruptured and an arc is established within the groove 11.

An alternative form of confining structure for the fuse element is shown in Figs. 3 and 4. A central insulating member, which may be of material similar to that designated by the numeral 8 in Fig. 1, comprises two members 14 and 15. Longitudinal slots 16 are disposed in the face of one or both of the members 14 and 15 and when the latter are held tightly together by the outer casin 110 2, tight joints are formed which separate the longitudinal slots. Other details of the structure shown in Figs. 3 and 4 are similar to those of Figs. 1 and 2 and require no separate description, except that the fuse element- 5 has an insert 5' of high-resistance material, such as tungsten, for a purpose to be described later.

Figs. 5, 6 and '7 disclose another embodiment in which a central core, similar to 8 in Fig. 1, is divided into three sections 17, 18 and 19. Longitudinal slots 21 are formed in both surfaces of the member 18 and are joined by apertures 22 passing through the same. The slots 21 on one surface are in staggered relation with those on the other surface and are connected by the apertures 22 to form a continuous recess. Such a construction enables the slots on any one side of the central core 18 to be separated by a greater distance than those shown in Figs. 3 and 4, although the diameter of the central core is the same in each case.

These several figures disclose different constructions that may be employed to compactly dispose a great length of fuse wire without occupying more space than a prior art fuse for a low potential system.

Such being the general structure of our fuse, reference may be had to the curves shown in Figs. 8, 9 and 10, for an exposition of the principles to be followed in proportioning the structures of any of the foregoing modifications. Thus, Fig. 8 shows the current-voltage relation of an unconfined arc in air, in which an increase in current will be seen to cause a decrease in voltage. In contrast to this, however, experiments have shown that when the arc plays in a confined space, the volt-ampere characteristics take the form shown by the curve AHGF in Fig. 9. We believe that the minimum point G of the curve occurs for the value of current at which the arc first entirely fills the recess within the insulating medium and that it is because of the confining action of the recess walls that an increase of current beyond this value results in the arc voltage curve having an ascending characteristic which rises rapidly with increase in current. Thus, in the curve AHGF in Fig. 9, a current increased to the value J corresponds to a voltage decreased to a value B. Beyond the point G, on the curve, however, at which it is probable that the are entirely fills the recess in the insulating medium, and can no longer expand with a further increase of current value, voltage will rapidly increase with further rise of current as illustrated by the portion GF of the curve. The point G represents the minimum voltage that can sustain an arc in the recess in question.

If, however, the recess should be so small that the filling thereof by the arc would occur at H, the current would have increased only to the value K when this deviation from the open are characteristic occurred. This corresponds to the higher minimum voltage indicated at C. A further increase in the current value that would result in a rapid increase of arc voltage is illustrated by the curve H1 in Fig. 9.

If the recess has a value intermediate between those just considered, so that the arc is confined at M, that is, at a point when the current value is between the above-mentioned current values at K and J, the increase of voltage would begin at the point M on the curve corresponding to a value L of voltage. Thereafter, an increase of current will cause the voltage to rapidly rise to 0.

It will be observed that the downwardly sloping portions AH, etc., in the volt-ampere curves under discussion correspond to a condition of electrical instability in a constant voltage circuit. Hence. when the fuse ruptures, the current will almost instantaneously seek 2t value somewhere on the rising portion of the volt-ampere curve beyond the minimum point thereof. It is clear also that substantially the entire line voltage will be impressed across the terminals of the fuse and that, accordingly, the current will assume that value on portion HI, MO or GF of Fig. 9 which corresponds to the line voltage. This fact, therefore, fixes the steady value which the current in the arc will reach when the fuse ruptures. This value will quickly fall to zero in the course of the alternating cycle and the arc will remain extinct.

The rate of disappearance of the ions in the arc path is so great because of the close proximity of the walls of the recess after the current passes through zero value, that the critical voltage gradient required to restrike the are between the terminals rises with extreme rapidity. For the usual commercial circuit, we find it easy to apply our fuse so that the rate of rise of its critical voltage gradient is greater than the rate at which the line voltage can rise, and so the arc remains permanently extinct.

Experiments have shown that the voltage per inch length of arc developed by an arc in a restricted recess varies inversely with the diameter of the latter. This relation is shown by the curve in Fig. 10 plotted from data obtained in tests and found to be represented by the equation where E d: C:

Diagonally across these curves for particular diameters of recess is shown a curve connecting voltage with diameter of the hole in which the arc is confined for an arc current limited by the confining walls to 1000 amperes. This curve may be employed to obtain the data for designing the fuse herein disclosed to meet a particular line condition. Thus, after the size of hole and voltage per inch length of are have been selected, the required length of the recess and fuse wire can be determined. As pointed out in the foregoing, if this length is too small, the power developed in the structure will cause it to deteriorate or to be destroyed. I

The foregoing considerations may be employed in designing fuses in accordance with our invention. Thus, it may be assumed that the voltage of the are within the groove 11 under practical operating conditions may at times be greater than peak value of the line voltage and less than approximately twice this peak value. Such an assumption makes allowance for surges and the effect of inductance present in the protected circuit.

Making such an assumption, Fig. 9 shows the value of line current to be expected with a given size of recess 11. It is also evident that if a confining recess of too small a diameter is employed. the rapid rise of voltage in combination with the current of the circuit and other characteristics would dissipate so much energy in the structure that it would deteriorate or even be completely destroyed. If too large a hole is employed, another difficulty is encountered and this will be explained, hereinafter.

We have found that if an alternating-current are is confined in narrow recess of small crosssection, comparatively high voltages per inch length of arc may be interrupteiil. Tests show that at 60 cycles in a 3/16 hole. an arc will be interrupted in a circuit having 280 volts per inch length; in a 2/16 hole 500 volts per inch may be interrupted, and in a l 'l6 hole more than 1150 volts per inch may be interrupted. However. we find that if the current in the arc in such a hole is too large, a very large disruptive pressure developed which the insulating material cannot withstand, 01" that so much heat is developed as to destroy the material. This necessitates the choice of a recess of such diameter that a destructive effect will not be produced.

Further, it is necessary that the arc voltage shall exceed the voltage of the line if the current is to be limited to a small value by the fuse. Hence, as small a recess should be employed as is possible, so that the arc is immediately confined upon the rupturing of the fuse element and the current will remain within moderate bounds while the arc voltage rises abruptly to a greater value than the line voltage.

Thus, when the fuse is ruptured, the rise of current is limited by the confining of the arc in the recess and the voltage of the arc rises above the voltage of the circuit. This peak voltage thereafter will immediately decrease, causing a corresponding decrease in the current.

The aforesaid formula, when the constant C is taken which corresponds to 1000 ampere current or less, takes the form This has been found satisfactory for recess diameters of less than 0.1. For greater recess diameter and larger current the formula may not hold but the foregoing may be taken as a suitable figure for practical purposes.

In order to solve for the values of 1, the length of recess and for (1, its diameter the following formulas have been evolved:

Where I is the value to which the shore circuit current has arisen when the fuse element is ruptured. Formula (d) is employed on circuits that have a low capacity and for circuits of high capacity, where large pressures are likely to be developed, the following formula should be employed instead.

Where 1 is the length and r is the resistance of the fuse wire.

By employing the foregoing formulas there may be obtained for a particular circuit condition the length and diameter of recess which will limit the pressures developed to values within the elastic limit of the soapstone structure. For structures of stronger insulating materials, no doubt the factor for the maximum safe power as above derived, will be considerably higher.

It will be noted in the curve, Fig. 10, that the voltage versus recess-diameter curve has been constructed for 1000 amperes. Other curves for less or greater currents should be constructed to fit the different circuit conditions, as for example, a high voltage circuit in which a current of only One or two ainperes are flowing. In this case, the fuse will usually be ruptured when the current had risen to not more than 200 ampercs. For this reason, care must be exercised in select ing a fuse wire of such calibration that on a very rapid rise of current. the wire will be ruptured at the desired current value.

If a greater current than that for which the structure is calculated is required to interrupt the fuse, a greater rise in voltage would result, causing a considerable increase in the pressure developed above that permissible for the strum ture that has been built from the data calculatcd from the above formulas.

Upon interrupting a circuit with our structure in transients that would ordinarily be produced by the rapid rise of current and voltage when a dead short" occurs in the system and which would ordinarily have a destructive effect upon the system or to apparatus connected thereto. are almost entirely eliminated. To obtain this limitation of the current rise, the proper recess diameter and length must be employed with a fuse wire of precise calibration. This fuse wire may be of copper, tungsten, or any metal or combination of metals as will be explained hereinafter.

When the fuse wire is interrupted, the voltage required to maintain an arc at the momentary value of current will exceed the voltage of the circuit. This causes the current to rapidly decrease until it has the value corresponding to the momentary voltage of the circuit as given by the curves of Fig. 10 or the Formula (1)). This current may be very much smaller than the dead short circuit current of the system. and thus the limiting of the current takes place, which is the major purpose of this invention. This reduction of the current when the fuse element is interrupted transpires in a very short period of time, a fraction of a half cycle of the alter hating wave. As soon as the voltage value reaches zero in the course of its alternating cycle, the current has also decreased the last small amount so that the arc is immediately extinguished when both these values reach zero.

The rate of re-combination of the ions in the arc path is so very greatly increased by their close proximity to the cool walls of the recess, that after the current passes through zero value, the voltage-gradient required to re-strike the are between the terminals rises extremely rapidly. For the usual commercial circuit, we find it easy to apply our fuse so that this required rate of rise of voltage is greater than the rate at which voltage supplied from the line can rise and so the arc remains permanently extinguished.

In addition to the voltage shown in Fig. 10, and given by Formula (22) we have found that there is at the moment the fuse wire vaporizes, a con siderably higher voltage which may be troublesome in practical circuits. This over voltage lasts for only a few milliont-hs of a second. and probably exists only during the time that the current spreads from the cross-section of the fuse wire to the cross-section of the recess. We have found that this excess voltage is least when tungsten wire is used and we therefore employ tungsten wire in place of other materials such as copper, wire or nickel silver wire as with the latter the excess voltage may be from three to nine times the sustained arc voltage.

The excess voltage referred to here occurs when the fuse wire blows substantially all at once. We may reduce the excess voltage by causing the fuse to blow in a more gradual manner. We do this by constructing a short portion of the fuse wire of. high resistance wire and the rest of the fuse of low resistance wire as in the modification shown in Figs. 3 and 4.

When such a fuse wire is subjected to a rapid rise of current, the high resistance portion of the fuse wire ruptures first, starting a short arc. The ends of the low resistance wire then burn away, lengthening the arc in a gradual manner. When the arc reaches a length such that its voltage-is greater than the supply voltage, the current begins to decrease so that by the time the arc has reached the full length of the recess, the current is so far reduced that only a moderate rise of voltage takes place. Thereafter, of course, as pointed out above, the voltage and current will drop to small values and as the voltage passes to zero value, the current will also reach zero and the arc will be extinguished.

Accordingly, our invention comprises a fuse for high potential alternating current circuits that is constructed to limit the current and interrupt the circuit within a half cycle of alternating current and to prevent the arc to restrike thereafter. Our fuse is rated according to the line conditions and by choosing the diameter of the recess and the proper length thereof according to these conditions, our fuse will limit the short circuit current and have no difficulty in permanently interrupting the circuit within the time of a half cycle of the alternating-current wave.

Our fuse is further novel in that liquid cooling medium is not required and also in the very low cost of producing such a fuse structure and in the ease by which our fuse structure may be renewed.

We do not wish to be restricted to the specific arrangement of parts herein set forth as various modifications thereof may be effected without departing from the spirit and scope of our invention. We desire, therefore, that only such limitations shall be imposed as are indicated in the appended claims.

We claim as our invention:

1. A fuse for a high potential circuit having a fuse element in series therewith, said fuse element comprising a portion of high-resistance material joined to a portion of a material having a lower resistance, a confining insulating channel enclosing the fuse and of such diameter and length that the current is limited in value and of such diameter and length that with this limited current the power developed per unit length will not be sufflcient to rupture the confining structure.

2. In series with an electric circuit, a fuse for theinterruption thereof having a fuse element having a portion of high-resistance material in series therewith, an insulating member having a continuous uniform recess for the fuse element associated therewith constructed in such manner as to reduce the overall length thereof, the said recess having such diameter and length that the current and voltage rise in the are established in the recess after the fuse is ruptured will not exceed a predetermined amount.

3. In series with an electric circuit, a fuse for limiting the short circuit current thereof, the fuse element of which comprises a short portion of tungsten joined to a metal of lower resistance, said fuse element being positioned in a container having terminals to which the ends of the fuse element are connected.

JOSEPH SLEPIAN. RURIC C. MASON. 

